I happened to want to order something there last Sunday. While digging into it, I notice that their closure seemed to be implemented only on the frontend using JS. By disabling some scripts, I was able to add products to my cart, which normally isn't possible on Sundays.

In the end, I didn't proceed to place my order on Sunday. It felt a bit disrespectful, and it was already late. (So I ordered half an hour later, which was technically Monday.)